HOLT: The beta deficit – why portfolios can't keep pace

A small group of mega-cap growth companies - the largest companies in the stock market, often worth hundreds of billions or even trillions of dollars - now accounts for an increasing share of market performance. At the same time, the rest of the market is moving less in unison than it has in the past, with individual stocks and sectors behaving more independently.

The result is a market that may appear well diversified on the surface but is increasingly being driven by a relatively small number of companies.

As market leadership becomes more concentrated, diversified portfolios may find it harder to keep pace with benchmark returns. In a simulation of 10,000 actively managed portfolios invested in large companies, the average portfolio beta - a measure of how closely a portfolio moves with the overall market- was 0.92. Only 16% achieved a beta of 1.0 or higher, meaning most portfolios were less sensitive to market movements than the benchmark index. Nearly half finished below 0.90.

When a small number of companies generate a large share of the market's gains, investors who spread their investments more broadly may not fully participate in those returns. In this environment, diversification can sometimes act as a headwind rather than an advantage.

The same factors that have supported today's market leaders have also created larger differences in valuations across the market. In other words, while some companies have become increasingly expensive, others have lagged behind and may now offer more attractive entry points for investors than they have for much of the current market cycle.

As market concentration remains elevated, understanding how market structure affects portfolio performance is becoming increasingly important. Market gains are no longer being shared evenly across a broad range of companies, creating a growing gap between benchmark returns and the experience of many diversified portfolios.